First thing I'd check is if the printer is out of color ink - if the printer doesn't have an indicator on the printer itself (don't know off hand about your model), go to system preferences-> printers & scanners -> select the printer and select the options/supplies button and under the supplies panel see if it's out of color.

Next thing I'd probably do is when I pull up a print dialog, where the dropdown says layout, on my Mac, if I hit that I have an option Color/Quality where there is a check box that can be marked as print color as gray and make sure that's not checked.